[docs] make DateTimePickerValidation example work out of the box

[dandv]

That example generates warnings in the CSB console though (#1776).

The mask "//____ : _M" you passed is not valid for the format used P p. Falling down to uncontrolled not-masked input.

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[cypress]

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Test summary

77 • 0 • 1 • 0

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Run details

Project	material-ui-pickers
Status	Passed
Commit	ac0dbf8
Started	May 25, 2020 5:44 AM
Ended	May 25, 2020 5:46 AM
Duration	01:39 💡
OS	Linux Debian - 9.11
Browser	Chrome 78

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[dandv]

Also, I can't select any time - #1818.

[dmtrKovalenko]

We are not going to add the <LocalizationProvider /> in each demo. At least for now, I think it is only adding white noise to the examples.

[oliviertassinari]

make DateTimePickerValidation example

@dandv We have decided to remove the built-in validation to leave more freedom to users #1730.

That example generates warnings

See #1776.

Also, I can't select any time - #1818.

Perfect, if you could open a new issue for each problem instead of "bundling" them, it would be much better.